He started out by sourcing a pair of solar panels from DealExtreme that purported to supply 5.5v @ 80mA. Like most of us are inclined to do, he tested them before use and found that they actually put out around 50mA instead. While the performance was a bit off, they still fit his needs pretty well, as the charge current needed to be at or less than 100mA to avoid damaging the battery.

He opened the Kobo’s case, and carefully removed a section of the back panel to make room for the solar panels. Once they were soldered together in parallel, he wired them to the eReader’s battery through a Schottky diode to prevent the battery from draining.

While we think his solar modification is a great way to ensure that he never runs out of juice while reading by the pool, we would certainly add a bit of extra charge circuitry to ours to prevent damage to the battery. What do you think?

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22 thoughts on “eReader battery never goes flat (in the sun)”

well …fine, but….as well as for the example of the mobilephone: Don’t understand the need for cutting the case in order to put solar cells. why not just using adhesive velcro strips over a thin and small external platform?!!?

The thing all you guys are forgetting is that these things only draw power when they need to refresh the display. E-ink doesn’t require constant refresh rates. It can get JUST enough charge to refresh the display while he’s reading a page, therefore allowing him a continuous stream of reading in his hammock on a sunny day, even while running low on battery. This obviously isn’t meant to be a main charging method, but rather a battery extension method.

And the fact that once again they used too small and B grade cells. for that e reader to charge at a DECENT rate the entire back needs to be covered and they should have used higher grade solar cells that put out far more power.

http://www.goldmine-elec-products.com/prodinfo.asp?number=G17096 for starters is a good choice. to get a decent charge rate of 5V at 2.5 amps these would need to be used in an array to deliver the 5 volts. (I would go to 6 volts and regulate down to give a decent charge rate in non optimal light conditions. best would have been a fold out panel, flip over the e reader and flip open the panel to get 2X the e reader’s surface area for charging.

Thus is the problem. Go tiny solar so you can charge it in a few days, or go large solar and get a full charge in 2 hours.

At least cover the entire back in solar. Come on guys, go all the way with these solar projects!

The thing is that if you’re reading it, the cells are in shade, and it’s not like you can read e-ink at night (because desert islands have no electricity or bright enough fires or even Tiki torches good enough to read by).

So I guess it’ll never get overcharged unless you abandon it face down on a beach :3 But if you do, it’ll eventually explode :|

I think I’d rather modify a folder-style carrying case, though. You could put more cells on the inside of the front cover, and the unit could charge while you were reading. If you got a case with a reading light, you could get rid of the light and modify the battery holder to accept the charge from the cells, then provide it to the reader.

Is anyone else uncomfortable with trickle charging a lithium ion battery? Proper li-ion chargers are both current- and voltage-regulated and have a low current cut off.

Relying on the battery’s high voltage cutoff seems like a bad idea since it the cutoff is necessarily higher than recommended charging voltage and it allows trickle charging, which is not acceptable for these batteries.

Yeah a li-ion cell maximum charging voltage is 4.2V and most charge regulators are spec that at 0.5% or 1%. My understanding is that this is ~5 in the right sun minus the diode drop which changes by batch, current and temperature. That’s just asking for the battery to die or otherwise (explode).

I’m not sure I understand why people are worried about trickle charging.

When you plug in your eReader into a wall outlet charger, its not the wall wort that terminates the charge, its the device. As long as this cells are providing power through the normal charge path, its a non issue. (Granted I can’t check the original site cause it appears to be down)

The normal charging path (USB) might require more current to operate reliably than solar cells can provide.

I wonder though if you could, in addition to the schottky, add a zener as a shunt to limit the max voltage of the solar cells. With a 1N4731 4.3V zener, you’d see about ~3.9V across the battery. Although it won’t be able to fully charge the battery, it should keep it operating indefinitely (given enough sun) without risk of battery damage.

A neat idea, but hasn’t anyone noticed that the cells are on the back, which usually faces DOWN when you are reading? And I’m not just going to leave an ebook lying in the sun by itself, it’ll grow legs pretty fast.

These sissy cells will not put out anything unless pointed directly at the sun, really.

Better to just use a solar->usb charger, put it facing the sun and plug the ebook in. You get regulated charging, no holes in your ebook reader, and probably around the same price with no risk or effort. I don’t usually say it, but this hack is lame.

@bothersaidpooh
No the battery would only be 20% – 40% charged at 4.0V http://www.limn2o4.com/battery/charger2.jpg Okay so it’s for a LiMn2O4 battery but the curves are very similar. You need to hold the battery voltage at 4.2V (Not 4.0) until the current has decreased substantially for a complete charge.